Attention is directed to an article by the inventors and a colleague entitled "Efficient GaAs/Ge/Si Solar Cells" in the Proceedings of the Sixteenth IEEE Photovoltaics Specialists Conference, pp. 1143-1148 (September 1982) and an article by Fan et al. entitled "Optimal Design of High Efficiency Tandem Cells" from the same proceedings at pp. 692-701; both articles are incorporated herein by reference.
By way of initial definition it should be noted that much of modern semiconductor technology makes use of thin solid films on the surfaces of solid substrates. A number of methods have been used to deposit such thin films including thermal evaporation, DC sputtering, RF sputtering, ion beam deposition, chemical vapor deposition, plating, molecular beam deposition and deposition from the liquid phase.
The structure of thin films can be amorphous (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, but the small regions have no mutual alignment of their crystallographic axes), textured-polycrystalline (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, and one or more of the crystalline axes of the majority of said regions are parallel), or epitaxial (that is, the film is predominantly of a single crystallographic orientation). An epitaxial or nearly single crystal film is a special case of a preferred orientation film in which corresponding crystallographic axes of all the small regions are essentially oriented in the same directions. A thin film can be the same material (that is, the same element or compound) as the substrate (producing a "homogeneous" structure), or it can differ in chemical composition from the substrate (producing a heterogeneous structure). If the film is epitaxial, the former is called "homoepitaxy" and the latter "heteroepitaxy".
By "crystallization" is meant the process of arranging the atoms of a substance in a crystalline order. For convenience, the term should also be understood to encompass "recrystallization" as well, when referring to a substance which already has some degree of crystalline order, in which case, the atoms are arranged in a higher crystalline order by "recrystallization".
In the pursuit of low-cost, efficient solar cells, for example, there is currently strong interest in depositing various semiconductor films on low-cost substrates. However, when the total costs of photovoltaic systems are considered, the balance of system costs are normally so high that highly-efficient cells are preferred. In other applications, such as semiconductor lasers and integrated circuits, high quality semiconductor layers are also needed.
Because of cost and efficiency consideration, one is frequently confronted with trying to deposit high-quality semiconductor layers on substrates significantly different from the layer, in crystal structure, lattice constants and thermal expansions--factors which are instrumental in determining whether good layers can be obtained. The most critical problem of growing such layers on dissimilar substrates is the lattice mismatch between the materials, which causes a large density of dislocations in the resultant grown layers. To reduce the dislocation densities, various ideas have been proposed, ranging from graded interface alloys (for example see J. C. C. Fan, C. O. Bozler and R. W. McClelland, Proc. of 15th IEEE Photovoltaic Specialists Conf., June 1981, Florida, p. 666 and U.S. Pat. No. 4,357,183 issued to Fan et al.) to superlattice techniques (for example, see K. W. Mitchell, Proc. of 15th IEEE Photovoltaic Specialists Conf., June 1981, Florida, p. 142).
In U.S. Pat. No. 4,246,050 a method is disclosed for growing crystalline layers from the liquid phase employing a stepwise cooling procedure such that dislocation defects are more uniformly distributed over the surface of the growing layer.
While the proposals for reducing dislocations have varying degrees of merit, there still exists a need for simple methods for reducing dislocation densities in grown layers, particularly in crystalline layers grown from a vapor phase.